1. Field of the Invention
The present invention relates to a touch panel device including an input pen.
2. Description of the Background Art
In order to detect an input position on a touch panel, there are provided various types of touch panels, such as resistive touch panels, infrared scan touch panels, surface acoustic wave touch panels, acoustic pulse recognition touch panels, touch panels with infrared image sensor and capacitive touch panels. Of those, the capacitive touch panels are classified into a projected capacitive touch screen (hereinafter, referred to as PCT) and a surface capacitive touch screen, in which an input position of a finger is detected by detecting a capacitance change between the finger of an operator and a touch panel detection electrode wire.
In the PCT, it is possible to detect touch even when a front surface side of a touch screen containing a touch sensor is covered with a protective plate such as a glass plate whose thickness is approximately several mm. The PCT has advantages in that a touch screen has excellent toughness, that touch detection is enabled even by a gloved finger, and that the touch panel has long life because a movable part is not provided.
There are various ways of detecting touch capacitance of a touch panel of the PCT, and for example, there is a touch panel using a self-exciting oscillator circuit as described in National Publication of Translation No. 09-511086 (1997).
The touch screen described in National Publication of Translation No. 09-511086 (1997) is configured to include, as detection conductors for detecting capacitance, a first series of conducting material pattern (conductor elements) formed as a thin dielectric film and a second series of conducting material pattern formed through separating an insulating film, have no electrical contact between the conductors, and include a plurality of intersections formed therebetween. In addition, as shown in FIG. 8 of National Publication of Translation No. 09-511086 (1997), the conductor elements which detect capacitance are connected to an oscillator 85 via a multiplexer 75 and an output line 72. An oscillating frequency of the oscillator 85 changes in accordance with parasitic capacitance of the conductor element and the touch capacitance formed by the conductor element and the finger, and outputs thereof are counted by a divide-by-n circuit 90 to be made capacitance detection data.
Further, Japanese Patent Application Laid-Open No. 08-137607 (1996), Japanese Patent Application Laid-Open No. 08-147092 (1996) and Japanese Patent Application Laid-Open No. 08-95701 (1996) disclose devices which perform finger input and pen input by switching input detection. Japanese Patent Application Laid-Open No. 08-179887 (1996) and Japanese Patent Application Laid-Open No. 11-305932 (1999) disclose input detection devices which are integrated with liquid crystal display (LCD) and simultaneously perform display and input.
However, for performing input to the touch panel of the PCT as disclosed in Japanese National Publication of Translation No. 09-511086 (1997), the operator needs to touch the touch screen with a finger. The input with a finger has advantages of, for example, high operability and simplicity, as well as high safety because the touch screen does not respond to objects other than a finger. On the other hand, an area of the finger per se is larger compared with a pen's tip, and hence it is difficult to press a small area or input a character by inputting with a finger. Therefore, it is impossible to input a position with high definition by inputting with a finger.
A contact area of the finger tip differs depending on age, sex, body size and a manner of touching the touch screen, but it is approximately 100 mm2. An electrode touched with the finger and the detection electrode interpose protective glass or the like therebetween. Therefore, the capacitance formed between the electrode touched with the finger and the detection electrode differs depending on a structure of the touch screen, and it is generally several picofarad (approximately from 2 pF to 5 pF). In order to input a position with high definition, suppose that stylus pen with a conductivity electrode which is much smaller than an area of a finger tip at the pen tip and is electrically connected to a human body is prepared temporarily. Then, even when the stylus pen touches a touch screen, the capacitance formed by the touch screen and the pen tip electrode is considerably small in proportion to an area of the touched electrode, and hence it is extremely difficult to detect a position thereof.
Here, it is assumed that a dielectric constant is ∈0, a relative dielectric constant of a material (protective glass) located between the detection electrode and the touch surface is ∈s, a finger touch area is St, a stylus pen contact area is Ss, and a distance (gap) between the detection electrode and an electrode for forming a capacitance at an input position is d. Then, in each case, where the touch screen is touched with a finger and where the touch screen is touched with the stylus pen, the capacitance formed between the touched electrode and the detection electrode are represented as follows when calculated under a simplification by using a plate capacitor model.
Case where the touch screen is touched with a finger: Ct=(∈0×∈s×St)÷d
Case where the touch screen is touched with a stylus pen: Cs=(∈0×∈s×St)÷d=Ct×(Ss÷St)
The dielectric constant ∈0, the relative dielectric constant ∈s and the distance d are the same in the two equations, and the capacitance are proportional to an electrode area of an input device such as a finger or a pen tip. Therefore, it is extremely difficult to perform position detection with the stylus pen having a small contact area.
Further, it is possible to realize position detection by combining with the input detection system which enables stylus pen input as disclosed in Japanese Patent Application Laid-Open No. 08-137607 (1996), Japanese Patent Application Laid-Open No. 08-147092 (1996) and Japanese Patent Application Laid-Open No. 08-95701 (1996). However, in such position detection, there arise problems of an increase in cost for modifying structure of the touch panel, quality degradation in LCD display due to a decrease in transmittance, and degradation in operability and simplicity. For example, since the input system is used in combination, there is required a switching circuit for selecting a detection function in advance. The methods disclosed in Japanese Patent Application Laid-Open No. 08-179887 (1996) and Japanese Patent Application Laid-Open No. 11-305932 (1999) are not particularly aimed to improve position detection with a stylus pen.